Inverters are often used in variable-speed, constant-frequency power generating systems to convert DC power produced by a brushless, synchronous generator and a rectifier bridge into AC power for energizing one more AC loads. A pulse-width modulated (PWM) inverter for producing three-phase AC power typically includes six controllable power switches connected in a bridge configuration. The switches are operated by PWM operating waveforms to produce a set of three PWM output voltages at junctions between the power switches. The output voltages switch between three voltage levels, i.e. zero volts, a positive voltage and a negative voltage, to produce a PWM waveform having a substantial fundamental component and higher harmonic components. Regulation of the output power produced by the inverter can be effected by sensing a parameter of the output power and selecting PWM switch operating waveforms in dependence upon the sensed parameter to in turn cause a selected parameter to approach a regulated value.
Stepped waveform inverters utilize a plurality of subinverter bridges coupled to a summing transformer. In a specific type of stepped waveform inverter, three-phase outputs of four subinverters are coupled to respective three-phase primary windings of the summing transformer. The windings of two of the sets of primary windings are connected in a wye configuration while the windings of the remaining sets of primary windings are connected in a delta configuration. The summing transformer further includes a set of three-phase secondary windings which are magnetically linked to the sets of primary windings. In operation, rectangular voltage waveforms are supplied to the primary windings by the subinverters to in turn produce a set of three-phase summed output voltages in the secondary winding. The output voltages comprise 24-step waveforms having a substantial fundamental component and higher harmonic components.
Each of the PWM and stepped waveform inverters has advantages and disadvantages. The PWM inverter requires only six power switches to produce a usable high power output. As a result, a relatively simple control unit may be employed to operate the switches and cooling requirements are minimized. However, the PWM inverter generates harmonics of sufficient amplitude to require the use of a large and heavy filter. This filter undesirably increases the size and weight of the overall inverter system and may render the system unsuitable for certain applications where size and weight must be minimized, such as in aircraft.
In addition to the foregoing, the number of switch transitions per cycle is occasionally high enough to cause power dissipation to increase unacceptably. Further, the inverter occasionally operates in an unreliable fashion when the number of switch transitions per cycle becomes too high.
The stepped waveform inverter generates substantially lower magnitudes Of harmonic content than the PWM inverter, and hence the filter size and weight are greatly reduced as compared therewith. Also, isolation between input and output is accomplished by the summing transformer and hence a separate isolation transformer is not required for those applications where isolation is needed. Further, EMI is reduced by the transformer. However, while the switches of the stepped waveform inverter can be lower power devices that are relatively inexpensive and readily available, a substantially greater number of power switches must be used as compared with the PWM inverter, and hence gate drive complexity and packaging requirements are increased. Further, the summing transformer size-and weight are not negligible, in turn partially offsetting the decreased size and weight of the filter.
The stepped waveform inverter has the further disadvantage in that the same number of switches must be used regardless of the output power level produced thereby.
Klein, U.S. Pat. No. 3,979,662 discloses an inverter system wherein the outputs of first and second inverters are coupled by first and second transformers, respectively, to a common load. More specifically, three-phase outputs of the first inverter are coupled to a set of wye-connected three-phase primary windings of the first transformer. Similarly, three-phase outputs of the second inverter are coupled to a set of wye-connected three-phase windings of a second transformer. The first transformer includes a set of three-phase secondary windings whereas the second transformer includes a set of three-phase secondary windings and a set of three-phase tertiary windings. The phase A primary winding of the first transformer is connected by the phase B secondary winding of the first transformer, the phase A secondary winding of the second transformer and the phase C tertiary winding of the second transformer to the load. In like fashion, the phase B primary winding of the first transformer is connected by the phase C secondary winding of the first transformer, the phase B secondary winding of the second transformer and the phase A tertiary winding of the second transformer to phase B of the load. Further, the phase C primary winding of the first transformer is connected by the phase A secondary winding of the first transformer, the phase C secondary winding of the second transformer and the phase B tertiary winding of the second transformer to phase C of the load. The inverters are operated to produce pulse-width modulated waveforms which are vectorially added by the transformers to produce stepped waveforms having pulse-width modulated notches therein.
Paice, U.S. Pat. No. 4,698,739 discloses a motor drive wherein the outputs of first and second inverters are coupled to primary and secondary windings of a transformer. The inverters are operated to produce a stepped waveform in the inverter output.